In many systems, it is desired to move a driven component along rectilinear, such as X and Y coordinate axes. Reference may be made to U.S. Pat. Nos. 3,851,156; 3,970,841; 3,999,047; and 4,061,914 all relating to an autofocussing system. The aforementioned patents disclose apparatus for differentially classifying white blood cells, wherein a plurality of sample cells are placed on a laboratory slide. The laboratory slide is incrementally moved in both X and Y direction to sequentially position each cell with respect to a microscope or other analyzing means.
If it is desired to increase the system throughput, i.e., the number of cells or samples which can be processed per unit time, it is necessary to reduce as much as possible the time required to position the driven component to move the cell to the desired location. The accuracy of movement at the desired increased speed must, or course, be preserved. Components presently available in the prior art may be utilized to achieve the increased speed and yet maintain accuracy of movement. As an example, low friction linear bearings are available having two rail members and a strip of retained rollers mounted therebetween for supporting the driven component during movement. In addition, a ball nut drive assembly affixed to the driven component and incrementally moved along a stepping motor shaft, when combined with the aforementioned prior art low friction linear bearings provide the speed of movement and the accuracy desired.
However, it was found that the system throughput was still restricted in that the driven component or stage upon reaching the desired location continued to vibrate and did not settle within the time required. "Settling" herein is defined as the condition of vibration in which there is no more than 0.5 micron of stage movement. Such undesired vibrations produce erroneous cell readings. One obvious solution is to allow the driven component or stage sufficient time to settle until the stage vibration is within an acceptable limit. Allowing the stage to settle until such a limit was obtained would have drastically reduced the system throuhput and is not compatible with the increased speed and accuracy of movement obtained.
Attempts were made to introduce friction into the system to reduce stage settling. This was found to adversely effect the accuracy or precision in stage movement. Likewise, when friction in the system is reduced to allow for greater accuracy of the movement, the stage settling times increased. It is thus desired to provide a rectilinear drive apparatus which can meet very stringent requirements and very accurately to a desired location with an absolute minimum of time required to reach stage settling.